A method has been known in which light enters an organism from a light source placed on the surface of the organism. The light appears again on the surface of the organism after propagating through the inside of the organism while being scattered and absorbed. The light received on the surface is used to measure the concentration of an absorbing material in the organism or the thickness of a tissue. FIG. 38 shows a positional relation between a light source and a light receiving element and an organism in a subcutaneous fat thickness measuring apparatus described in Japanese Patent Laid-Open No. 2000-155091 as one example of the method. The disclosure of Japanese Patent Laid-Open No. 2000-155091 is incorporated herein by reference in its entirety. A light source 302 and a measuring light receiving element 303 are placed on the surface of an organism 301. Given that the organism has a structure of a parallel flat plate having three layers of a skin 305, a subcutaneous fat 306 and a muscle 307 as shown in FIG. 38, light 308 received by the measuring light receiving element 303 has a correlation with the thickness of the subcutaneous fat 306 due to a difference in absorption and scattering characteristics between organic tissues. However, the amount of light 308 received by the measuring light receiving element 303 varies under significant influences of changes in blood flows of the skin 305 and the subcutis. Therefore, a correcting light receiving element 304 is placed near the light source 302 (1 to 6 mm from the light source 302), and light 308 received by the measuring light receiving element 303 is corrected by the amount of light 309 received by the correcting light receiving element 304, thereby making it possible to measure the thickness of subcutaneous fat with high accuracy.
However, because the organic tissue is not strictly a parallel flat plate as shown in FIG. 39, and arms and legs have cylindrical shapes as shown in FIG. 40, the measurement accuracy is compromised by a local change in thickness.
Also, because the organic tissue is soft and hence highly deformable, the shape of the surface of the organism 301 varies for each measurement even in the same person and the same site, and therefore the amount of received light is varied to compromise reproducibility.
Also, in the case where the subcutaneous fat 306 is thick, the distance between the light source 302 and the measuring light receiving element 303 should be increased for receiving light propagated through a deeper part in the organism by the measuring light receiving element 303. Therefore, there is a disadvantage that the measuring apparatus is scaled up.
Also, in the case where the subcutaneous fat 306 is thick, the distance between the light source 302 and the measuring light receiving element 303 is increased, and therefore the amount of light received in the measuring light receiving element 303 is reduced to compromise the measurement accuracy.
Also, in the case where the subcutaneous fat 306 is thick, the light reception sensitivity in the measuring light receiving element 303 should be improved, and therefore the accuracy and sensitivity of the measuring light receiving element 303 should be enhanced, thus raising a disadvantage that expensive parts are required.
Also, in the case where the subcutaneous fat 306 is thick, light incident from sources other than the light source 302 such as sunlight into the organism is measured even if the sensitivity of the measuring light receiving element 303 is improved, and therefore the surface of the organism 301 should be shielded sufficiently.
Also, in the conventional subcutaneous fat thickness measuring apparatus, the thickness of the subcutaneous fat 306 is changed in association with the variation in contact pressure applied to the organism by the light source 302 and the measuring light receiving element 303 on the surface of the organism, and therefore the thickness of the subcutaneous fat 306 varies for each measurement to compromise measurement reproducibility. This problem is significant particularly when the subcutaneous fat is thick.
In addition, the subcutaneous fat 306 is deformed due to the contact pressure, and therefore the amount of blood in the subcutaneous fat 306 is changed to cause a variation in absorption characteristics by the blood in the subcutaneous fat 306. Consequently, the amount of light received in the measuring light receiving element 303 fluctuates to compromise measurement reproducibility.